CROSSOSTOMA. 345 
Family—TURBINIDAL. 
*¢ Shell spiral, turbinated or pyramidal, nacreous inside ; operculum calcareous 
and pauci-spiral, or horny and multispiral.’—S. P. Woopwarp. 
The above diagnosis includes the Turbinide, Trochidw, and Delphinulide of 
other authors, and, as far as Jurassic fossils are concerned, where the nacreous 
layer is generally destroyed and opercula hardly ever to be found, cannot be 
followed very closely. 
The withdrawal of Amberleya and the so-called “ Littorina ” reduces the number 
of shells remaining to be considered under this family. These are for the most 
part small. Whatever generic names are adopted, we recognise two groups, viz. 4 
group of smooth and a group of richly sculptured shells. 
Grour I.—Smooth Shells. 
The smooth and generally small Turbinidz of the Jurassic Rocks have been 
variously classified. The following are amongst the genera which have been 
invoked: Ataphrus, Crossostoma, Chrysostoma, Monodonta, Pleuratella, Plocostylus, 
Rotella, Teinostoma, Trochopsis, Trochus, and Turbo. 
In the Inferior Oolite of this ‘country I think that the smooth Turbinidz may 
be focussed, provisionally at least, under three genera. Apart from one or two 
forms which it may be convenient to arrange under Turbo (Section 1), the rest of 
the shells exhibit a sequence from the most depressed form—Crossostoma Pratti, 
to the most acutely trochiform Monodontoid, viz. Ataphrus Acis. Mr. Tawney, it 
may be remembered, only recognised two species in his ‘ Dundry Gasteropoda,’ 
viz. “ Monodonta” levigata, Sow., and “ Monodonta” Acmon, a’ Orb. 
Genus—Crossostoma, Morris and Lycett, 1851. 
** Shell thick, twrbinated, smooth, somewhat depressed or rotelliform ; whorls 
more or less flattened, few; apex of the spire obtuse, depressed; aperture nearly 
circular, entire ; the columella forms at its base a simple prominent obtuse tooth ; the 
outer lip is smooth ; there is no umbilicus. . . . .’—Morris ano Lycerr. 
Lycett, in notes and corrections to the ‘Supplement,’ p. 153, withdrew 
